import java.util.Scanner;

public class Main1717 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        double x=(int)n;
        if (x==1){
            System.out.println(String.format("%.1f",x*5.0));
        }else if(x>=2&&x<=5){
            System.out.println(String.format("%.1f",x*4.5));
        }else if (x>=6&&x<=10){
            System.out.println(String.format("%.1f",x*4.0));
        }else if (x>10){
            System.out.println(String.format("%.1f",x*3.5));
        }
    }
}
